events: Update mouse-event handlers to not return EventResult.

ash/wm/activation_controller.cc

Index: ash/wm/activation_controller.cc
diff --git a/ash/wm/activation_controller.cc b/ash/wm/activation_controller.cc
index a17109ed57584852cff09bdd8bfca46add7584c8..29058f3f212863035321b699ba6fa560f02fdde4 100644
--- a/ash/wm/activation_controller.cc
+++ b/ash/wm/activation_controller.cc
@@ -275,10 +275,9 @@ void ActivationController::OnWindowFocused(aura::Window* gained_focus,
 void ActivationController::OnKeyEvent(ui::KeyEvent* event) {
 }
 
-ui::EventResult ActivationController::OnMouseEvent(ui::MouseEvent* event) {
+void ActivationController::OnMouseEvent(ui::MouseEvent* event) {
   if (event->type() == ui::ET_MOUSE_PRESSED)
     FocusWindowWithEvent(event);
-  return ui::ER_UNHANDLED;
 }
 
 void ActivationController::OnScrollEvent(ui::ScrollEvent* event) {